1. Field of the Invention
The present invention relates to a friction stir welding apparatus and a method therefor, and in particular, to a friction stir welding apparatus and a method therefor suitable for preventing weld defects such as voids and cracks which occur in weld or joint portions.
2. Related Art
For example, in Japanese Patent No. 2712838 (EPO 6158480B1) and Japanese PCT National Publication No. 508073/1997, there is described a friction stir welding method for performing a welding operation with friction heat generated between a metal rod and pieces to be worked by inserting the metal rod of a material substantially harder than that of the workpieces into the welding region of the workpieces and then moving this metal rod while it is rotated. This friction stir welding method utilizes a plastic fluidity phenomenon due to the friction heat between the metal rod and the workpieces, and it is based on a principle which is different from a method for melting and welding workpieces (e.g., arc welding or the like)
According to examination by the present inventors, a relative positional relation between the metal rod and the workpiece surfaces is important in order to carry out the friction stir welding method described in the above publications. The metal in a plastic fluidity state has to be prevented from overflowing onto the workpiece surfaces from the weld region. That is, if the metal flows out of the weld region, defects often occur in the weld region, which causes the reliability of the weld region to lower.
For the purpose of preventing such defects, it is required that in performing this friction stir welding method, the rotating tool is inserted into the workpieces to a certain depth from their surfaces and this depth is maintained also during the welding.
In general, however, if workpieces have rough surfaces, the height of the workpiece surfaces changes with feeding of the workpieces. In addition, also when the workpieces are heated during the welding, the height of the workpiece surfaces changes sometimes.
In such a case, a relative distance between the tool and the workpieces fluctuates, and the insertion depth of a shoulder portion of the tool which is inserted from the workpiece surfaces also fluctuates.
Further, if the surfaces of workpieces are not flat or if the heights of two workpieces differ from each other, a relative posture between the tool and the workpieces fluctuates on occasion.
An object of the present invention is to provide a friction stir welding apparatus and a method therefor where joining on rear sides of workpieces is improved and burrs are reduced by controlling a relative positional relation between a tool and the workpieces.
An apparatus according to the invention comprises a tool having a shoulder portion, a tool rotation mechanism for rotating the tool, and a support mechanism for supporting the tool rotation mechanism in a manner that at least one of the posture of a rotation axis along which the tool is rotated and the position of the tool in a direction along the rotation axis is changeable.
In addition, the aforementioned support mechanism makes changeable a relative positional relation between the tool and surfaces to be worked.
An apparatus according to another aspect of the invention further comprises a detecting section for detecting the relative positional relation between the tool and the surfaces of workpieces, and a control section for controlling the support mechanism so that the relative positional relation detected by the detecting section approaches to a predetermined relative positional relation.
An apparatus according to still another aspect of the invention further comprises a control section for controlling the support mechanism so that fluctuation of the relative positional relation detected by the detecting section is minimized.
The invention is further directed to a friction stir welding apparatus by which a tool of a material harder than workpieces fixed on a frame is inserted into a region to be joined of the workpieces, and they are joined by plastic fluidity of the workpieces, wherein the tool has a thread portion to be inserted into the workpieces and a shoulder portion supporting the thread portion and having a diameter larger than the thread portion, and the apparatus has push-in amount adjusting means for setting the thread portion to a desired length so that the thread portion does not contact a surface of the frame and for adjusting a push-in amount of the tool so that the shoulder portion forms a concave of desired depth on the surfaces of the workpieces. A distance between the tip of the tool and the frame is equal to or less than 0.4 mm, preferably from 0.05 to 0.20 mm. Therefore, the length of the thread portion is selected in compliance with the thickness of the joint portion of the workpieces. Further, the depth of the push-in amount is 1 mm or less, preferably from 0.2 to 0.6 mm.
The tool features having angular adjusting means for adjusting an inclination angle so that a leading part of the shoulder portion has a desired gap on the workpiece surfaces in the proceeding direction of welding, and adjusting the push-in amount by a trailing part of the shoulder portion. The inclination angle is within 10 degrees, preferably 3 to 7 degrees, in both directions with respect to a line perpendicular to the workpiece surfaces, and the frame features having grooves, preferably several grooves, for fixing the workpieces to the frame by hold-down members. The grooves are each preferably formed in the shape of a box so that the head of a bolt may engage therewith.
The apparatus features pressing means provided for pressing and restraining workpieces from their lateral sides so that the joint portion of the workpieces may not open.
The invention is accomplished by the apparatus that is equipped with shifting means for the revolution number, the rotation direction, the inclination angle, the vertical movement and the welding line direction of the tool, a control device for them, and hold-down means for the members to be welded, and that is manually or automatically driven. Moreover, the frame which holds workpieces is provided with the box-like grooves as described above, the heads of bolts are engaged in the grooves, and long arms are attached to these bolts, by which arms the workpieces are tightly secured with the aid of screws.
Further, it is preferable to provide a monitor device which can optically and electronically monitor a joining state in the course of welding. The center position of the tool can be adjusted through the monitoring.
The apparatus of the invention is preferably constructed as follows.
(1) The rotational direction of the tool in a forward path and a backward path for welding can be controlled by inverter control. The inclination angle can be adjusted by a driving force from an electric motor via an index shaft.
(2) The insertion depth of the tool from the workpiece surfaces can be adjusted by adjusting the depth of the tool in accordance with the unevenness of the workpieces or the shape of the welding depth. The insertion depth of the tool can be decided by changing the length of a pin from the shoulder. The length of the pin can be adjusted by establishing a set structure where the shoulder and the pin move mutually vertically and the rotations of them are the same.
(3) In the case of a joining state where a part of the surfaces of members to be joined are inclined, the inclination angle of the tool can be adjusted by automatically adjusting the driving means of the tool in accordance with the inclination angle of the workpieces.
(4) In the case that a plurality of joining lines are present on workpieces, a plurality of tools are mounted on the one friction stir welding apparatus and the revolutions, the moving speeds and the inclination angles of the tools are controlled independently or collectively, thereby achieving the joining efficiently.
(5) Workpieces can be restrained by the hold-down means disposed in the coaxial direction with the tool. Moreover, another independent hold-down means can also be provided separately from the first hold-down means.
The insertion depth of the tool from the workpiece surfaces can be adjusted by previously measuring the position of the unevenness from a welding start point on the workpiece surfaces and the change of the unevenness, allowing the control device to recognize the measured results, and then controlling a vertical driving device of the tool on the basis of the recognized signals. That is, a time taken for the tool to reach the position of the unevenness can be calculated from the position of the unevenness from the welding start point and the welding speed, so that a time taken to vertically drive the tool can be decided. Moreover, a difference of height in the uneven region can also be previously measured by a laser displacement gauge or a dial gauge, and the control device is caused to recognize the results. Even in the joining region where the uneven portions are partially present on workpieces, the depth of the tool can be controlled so as to always become constant from the uneven surface through the above technique. In the case that the unevenness is especially serious, the tool can be once pulled out of workpieces, so that the welding is interrupted, and the tool can be then inserted again in consideration of the uneven surface.
In the joining region where a part of the surfaces of workpieces is inclined, the inclination angle of the tool and the start point of inclination can be adjusted by previously measuring the inclination of the workpieces and the inclination angle at the start point, allowing the control device to recognize the results, and then controlling the driving means of the tool on the basis of the recognized signals. Furthermore, the positions of start and end of the inclination can be automatically judged on the basis of a time signal which can be obtained by calculating a distance from the welding start point. In addition, the inclination angle can be adjusted in the course of the welding operation on the basis of a signal which can be obtained by measuring the inclination angle as well as the start point and the end point of the inclination by the use of a laser displacement gauge in the course of the welding operation.
The invention enables joining on a forward path and a backward path in the welding process, and in this case, a plurality of tools, preferably 3 to 5 tools, are installed on one friction stir welding apparatus, and the revolutions, the moving speeds and the inclination angles of the plurality of tools are controlled independently or collectively, whereby the welding work can efficiently be accomplished.
Moreover, automatic adjustment of the tool angle can be achieved by conveying a drive force from an electric motor via a worm gear.
The automatic adjustment can be achieved by applying a pressure from the surface direction or the side direction of workpieces by oil pressure, hydraulic pressure or pneumatic pressure in accordance with the shape of the workpiece.
According to the invention, control can be accomplished so that the insertion depth of the tool from the workpiece surfaces may always become constant in the welding process and so that the insertion angle of the tool with respect to the workpiece surfaces may always become constant.
Accordingly, even in the case of a structure to be welded having a long weld region, occurrence of welding defects can be prevented, so that it is possible to make the welded structure with a high reliability.
According to the invention, for example, a weld structure which is made of an aluminum-based alloy plate and has a weld length as long as 20 m, more concretely, a railway car, especially a high speed railway car can be manufactured, preventing weld defects from occurring. Moreover, it is similarly possible to join an automobile body made of the aluminum-based alloy plate, and after the joining, finishing is done to a desired shape. After cutting, press working is performed to a desired shape.
Moreover, according to the invention, the rotation, the inclination, the vertical movement and the joining line direction movement of the tool in the course of welding can be optionally adjusted, and so the excellent joining of rear surfaces and the joining with less burrs on front surfaces can be accomplished. Further, because workpieces can efficiently and stably be held, stable joining can be accomplished even in the case of long workpieces, inclined workpieces or uneven workpieces.